Bimetallic Catalyst and Use in Xylene Production

ABSTRACT

The invention is directed to a bimetallic catalyst system adapted for the manufacture of xylenes, a process for making said catalyst system, and to the process of manufacture of xylenes using said catalyst system, providing, in embodiments, improved selectivity by at least one of higher ethylene saturation and low xylene loss, decreased susceptibility to poisoning from feedstream impurities, and ability to operate at less severe conditions.

PRIORITY CLAIM

This application claims the benefit of Provisional Application No.61/604,926, filed Feb. 29, 2012, the disclosure of which is incorporatedby reference in its entirety.

FIELD OF THE INVENTION

The invention relates to bimetallic catalysts used in the production ofxylenes, and more specifically in ethylbenzene dealkylation-xyleneisomerization systems to make paraxylenes.

BACKGROUND OF THE INVENTION

A typical aromatic hydrocarbon stream found in petrochemical plants andrefineries, such as may be produced by reforming and cracking naphtha,includes the C8 aromatic hydrocarbon isomers ethylbenzene and the xyleneisomers paraxylene, metaxylene, and orthoxylene. Paraxylene isrelatively high value as compared with the other isomers because it isused as the main raw material for polyester fibers. Orthoxylene, usefulsuch as for preparing phthalate esters for plasticizers, is relativelymore valuable than metaxylene. Unfortunately, an equilibrium mixture ofxylenes contains roughly twice as much metaxylene as para- ororthoxylene.

To recover paraxylene preferentially, typically a C8 aromatichydrocarbon stream is processed through a paraxylene recovery stage,such as an adsorption process (e.g., a Parex™ or Eluxyl™ absorptiveseparation unit) or crystallization process, to recover aparaxylene-enriched stream and a paraxylene-depleted stream. Theparaxylene-depleted stream can then be catalytically isomerized toequilibrium for recycle in the paraxylene recovery loop. Ethylbenzeneneeds to be removed from the loop and one way to do so is as explainedbelow.

Typically the catalyst used to promote isomerization of aparaxylene-depleted stream comprises a zeolite supported with a metalcomponent of Group 7-10 of the Periodic Table, e.g., platinum orrhenium. In addition to promoting isomerization between xylene isomers,ethylbenzene can be converted to benzene through a dealkylation reactionand subsequent hydrogenation of the coproduct ethylene, in the presenceof such catalysts. One of the undesired side reactions ismetal-catalyzed ring saturation and another is the production of C9+aromatic hydrocarbons. Research into increasing the efficiency of theparaxylene recovery loop is very active, and in particular there isconstant demand for a better catalyst.

Recent prior art related to isomerization processes includes U.S. Pat.Nos. 6,028,238; 7,247,762; 7,270,792; 7,271,118; 7,626,065; U.S. PatentPublication 2011 0190556; and U.S. application Ser. No. 13/081351.

Bimetallic catalysts per se are well-known and used in many differentrefining and petrochemical processes, such as in transalkylation in themanufacture of xylenes and other xylene manufacturing processes.

WO2009034093 (US2010/0217057) teaches a new configuration of ZSM-5having higher average silica to alumina ratio at the edges of eachcrystallite than in the center providing reduced xylene losses inethylbenzene dealkylation, especially when combined with silica asbinder and one or more hydrogenation metals selected from platinum, tin,lead, silver, copper, and nickel.

WO2007037866 (US2007/0060470) teaches a catalyst of certain combinationsof platinum, tin, acidic molecular sieve and aluminum phosphate binderfor isomerization and dealkylation activities with low naphthenes make.

U.S. Pat. No. 7,525,008 teaches isomerization of a C8 aromatic streamwith a MTW-type zeolite catalyst containing platinum and optionally tin.The MTW type zeolite has a silica to alumina mole ratio of between about20:1 and 45:1.

US 2011/0190556 teaches a xylene production process involvingtransalkylation of a C9+ aromatic hydrocarbon feedstock with a C6 and/orC7 aromatic hydrocarbon feedstock. The feedstocks are contacted in thepresence of hydrogen with a catalyst system comprising a firstbimetallic catalyst and, downstream thereof, a second bimetalliccatalyst. The first bimetallic catalyst comprises a molecular sievehaving a Constraint Index in the range of 3 to 12, such as ZSM-5, andthe metals are selected from Groups 6 to 12 of the Periodic Table. Thesecond bimetallic catalyst comprises a molecular sieve having aConstraint Index less than 3, such as ZSM-12, and the metals areselected from Groups 6 to 12 of the Periodic Table.

US 2010/0048381 teaches a catalyst for xylene isomerization including acarrier having a zeolite with a specified molar ratio of silica toalumina impregnated with or mixed with a metal salt, the carriersupported with a Group VIII metal, or a Group VII metal additionallysupported with tin, bismuth, or lead.

US 2007/0060470 teaches a catalyst comprising platinum and tin for theisomerization of xylenes and dealkylation of ethylbenzene. Ametal-containing molecular sieve having a silica to alumina ratio of atleast 20:1 is taught.

Additional references of interest include U.S. Pat. No. 7,271,118 (WO2006/022991); U.S. Pat. No. 7,199,070 (EP 1495805); U.S. Pat. Nos.5,689,027; 5,283,385; 4,485,185; and EP 2022564.

However, none of the systems described above are concerned withprocessing paraxylene-depleted feed streams through multiple bed systemswhere the functions of dealkylation and isomerization can be separatelymanaged for improved performance. Furthermore, increased metalselectivity (desired ethylene saturation versus undesired ringsaturation), decreased metal migration and decreased susceptibility tosulfur poisoning are still sought after.

The present inventors have surprisingly discovered a bimetallic systemfor a dealkylation and isomerization of a paraxylene-depleted C8aromatic hydrocarbon feed stream having low levels of at least one Group8-10 metal and at least one additional metal that provides, inembodiments, high ethylene saturation and low xylene loss at lowtemperatures when compared with current state of the art catalysts.

SUMMARY OF THE INVENTION

The invention is directed to a bimetallic catalyst system adapted forthe manufacture of xylenes, a process for making said catalyst system,and to the process of manufacture of xylenes using said catalyst system,providing, in embodiments, improved selectivity by at least one ofhigher ethylene saturation, lower xylene loss, decreased ring loss,decreased susceptibility to poisoning from feedstream impurities, andability to operate at less severe conditions.

In embodiments the catalyst system according to the invention comprisesa first bed comprising at least one first metal selected from Groups7-10, and at least one second metal selected from silver, copper,ruthenium, indium and tin, dispersed on ZSM-5, and a second bedcomprising at least one first metal selected from Groups 7-10, and atleast one second metal selected from silver, copper, ruthenium, indiumand tin, dispersed on supported ZSM-5, and to a process comprisingcontacting a paraxylene-depleted feed steam with said catalyst system inthe presence of hydrogen. The ZSM-5 in the first bed issilicon-selectivated and the ZSM-5 in the second bed is notsilicon-selectivated.

In embodiments the metals are added to the first bed by the IncipientWetness (IW) technique and to the second bed by mulling.

In embodiments the support is selected from alumina, silica,aluminosilicates, clay, and combinations thereof

In embodiments the process comprises contacting the first bed of thecatalyst system with a paraxylene-depleted feed comprising C8 aromaticsincluding ethylbenzene and xylene isomers, to convert a portion of theethylbenzene to benzene and ethane, to produce a ethylbenzene-depletedintermediate stream, and then contacting the intermediate stream withthe second bed of the catalyst system isomerize xylenes and provide afinal product comprising an increased amount of paraxylene and decreasedamount of ethylbenzene when compared with said feed.

In embodiments the catalyst system comprises a first bed comprising acatalyst made by adding at least one first metal selected from Groups7-10 and at least one second metal, different from said first metal andselected from silver, copper, ruthenium, indium and tin by the IncipientWetness (IW) technique to a selectivated ZSM-5 catalyst, and a secondbed comprising a catalyst made by combining a metal solution comprisingat least one first metal selected from Groups 7-10 and at least onesecond metal, different from said first metal and selected from silver,copper, ruthenium, indium and tin, and water and a ZSM-5 molecular sieveand mulling with a support such as alumina, silica, and combinationsthereof, to distribute the material. The resultant composition isextruded and then dried, calcined, and steamed. The ZSM-5 in the secondbed is not silicon-selectivated.

It is an object of the invention to provide a simple and reproduciblemethod of manufacture of a catalyst system adapted for the conversion ofethylbenzene and isomerization of xylenes that avoids the problems ofthe prior art with respect to xylene loss by ring saturation androbustness of the catalyst system with respect to sulfur poisoning andseverity of operating conditions.

These and other objects, features, and advantages will become apparentas reference is made to the following detailed description, preferredembodiments, examples, and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, like reference numerals are used to denotelike parts throughout the several views.

FIGS. 1-4 are plots comparing performance of bimetallic catalystsaccording to the present invention.

DETAILED DESCRIPTION

According to the invention, there is a bimetallic catalyst systemadapted for the manufacture of xylenes from a paraxylene-depleted C8aromatic hydrocarbon feedstream by dealkylation of ethylbenzene andisomerization of xylenes, a process for making said bimetallic catalystsystem, and to the catalyst system used therein. In embodiments, theinvention provides for improved selectivity by at least one of higherethylene saturation and lower xylene loss, decreased susceptibility topoisoning from feedstream impurities, ability to operate at less severeconditions, and ease of preparation of the catalyst system.

The catalyst system is a two-bed system with a first bed comprising acatalyst adapted for dealkylation of ethylbenzene and a second bedcomprising a catalyst adapted for isomerization. Each bed may be inseries in separate reactors or in the same reactor. The separatecatalysts are not mixed.

The invention may be better understood by reference to the followingdetailed examples, including experiments comparing catalysts accordingto the present invention with commercial catalysts of the prior art. Theexamples should be taken as representative of the present invention andnot limiting thereof.

Terms and techniques used below should be well-known to the skilledartisan and unless defined specifically herein take definitions found inthe prior art, particularly in those references discussed in thebackground or hereinbelow. Otherwise, The Handbook of Petroleum RefiningProcesses, 3^(rd) Edition; Robert Meyers, Editor; McGraw-Hill, ©2004,should be consulted before reference to other extraneous sources.However, for the avoidance of confusion the following description ofcertain terms is provided.

The Incipient Wetness (IW) technique is a well-known method ofincorporating metals into materials; see, for instance, U.S. Pat. Nos.4,302,359 and 7,271,118. Likewise, competitive ion exchange is awell-known method, such as discussed in the aforementioned U.S. Pat. No.7,271,118.

Mulling refers to mixing of materials with sufficient liquid to form apaste and wherein the mixing is accompanied by shearing of the resultantmixture. Commercially available mullers may be used. See, for example,U.S. Pat. No. 7,335,802.

Selectivation—in situ silicon selectivation is discussed, for instance,in U.S. Pat. No. 5,475,179; ex situ silicon selectivation is discussed,for instance, in U.S. Pat. No. 5,625,103. Other selectivation agents toincrease catalyst selectivity to paraxylene, such as carbon monoxide,are known; see for instance U.S. Pat. No. 7,902,414. The term“selectivated catalyst” as used herein means that the catalyst has beensilicon selectivated, either by in situ silicon selectivation or by exsitu silicon selectivation, by means per se known in the art. In thepresent invention the first bed contains a catalyst that has beenselectivated and the second bed contains a catalyst that has not beenselectivated.

Hybrid calcination per se means calcination at a desired temperature andfixed total flow of air and nitrogen with gradual decrease of nitrogenand gradual increase of air. At the end of the hybrid calcination, themajority of the flow is air.

Alpha value is a well-known measure of acid activity and gives therelative rate constant for the rate of normal hexane conversion pervolume of catalyst per unit time. See, for instance, U.S. Pat. No.8,057,664.

By “para-depleted” is meant a mixture of C8 aromatic hydrocarbons havingparaxylene present in an amount that is less than equilibrium value, theequilibrium value being approximately 23 to 24 wt % paraxylene based ontotal xylenes. A para-depleted mixture of xylenes results, for instance,from preferential removal of paraxylene by an adsorptive separationprocess such as by processing through a Parex™ or Eluxyl™ adsorptiveseparation unit, or by a crystallization process wherein the differencein freezing points of the C8 aromatic isomers is exploited. The term“para-depleted” is per se well known in the art.

Two-bed catalyst systems according to the present invention wereprepared. All top bed (or first bed) catalysts were prepared using asilicon-selectivated H-ZSM-5 as a base and adding metals by IncipientWetness (IW) technique. When two metals are used, the metal solutioncontained both metal sources. Tetraamine platinum nitrate was used asthe platinum source for all catalysts. After being dried in ambient air,all catalysts were calcined for 3 hours at 660° F. in 40% Air in N₂(˜8.4% O₂). Top bed catalyst Alpha values (hexane cracking activity) aretypically ˜450. The bottom (or second bed) catalysts were notsilicon-selectivated. To be sure, the designation “second bed” assumesthat the stream to be treated contacts the “first bed” first and thencontacts the second bed; likewise, the designation “bottom” bed and“top” bed assumes downward flow, top-to-bottom, although downward flowis not mandatory and other configurations can be envisioned by one ofordinary skill in the art.

TABLE 1 Top (First) Bed Catalyst Formulations 2nd Pt, Metal, 2nd Metal,Pt/M, Catalyst wt % (M) wt % Metal Precursor Molar Ratio E 0.10 None — —— F 0.10 Ag 0.0276 silver nitrate 2/1 G 0.10 Ag 0.0553 silver nitrate1/1 H 0.10 Ag 0.1106 silver nitrate 1/2 I 0.10 Ag 0.1659 silver nitrate1/3 J 0.10 Ag 0.2212 silver nitrate 1/4 K 0.10 Au 0.1010 Gold chloride1/1 L 0.10 Cu 0.0163 copper nitrate 2/1 hemipentahydrate M 0.10 Cu0.0326 copper nitrate 1/1 hemipentahydrate N 0.10 Cu 0.0651 coppernitrate 1/2 hemipentahydrate O 0.10 Cu 0.0977 copper nitrate 1/3hemipentahydrate P 0.10 In 0.0389 Indium (III) 1/1 Nitrate hydrate Q0.10 Pb 0.1062 Lead nitrate 1/1 R 0.10 Rh 0.0527 Rhodium Chloride 1/1 S0.10 Ru 0.0264 Ru Chloride 1/1 T 0.10 Sn 0.0304 tin (II) chloride 2/1 U0.10 Sn 0.0608 tin (II) chloride 1/1 V 0.10 Sn 0.1217 tin (II) chloride1/2 W 0.10 Zn 0.0335 Zinc nitrate 1/1 hexahydrate

Bottom bed (or second bed) catalysts were made by extruding 80% H-ZSM-5with 20% LaRoche Alumina Versal™ 300. Metal solution and water were alladded during mulling. For all catalysts except AG, the order of additioninto the muller was add crystal and mull to distribute, then add aluminaand mull, then add metal solution and mull, and finally add water andmull. For Catalyst AG, the order of addition was add crystal and mull todistribute, then add metal solution and mull, then add alumina and mull,and finally add water and mull. Catalysts were extruded to 1/16″cylinders. After extrusion, extrudates were dried, then hybrid calcinedat 1000° F. for 6 hours (80% Air, ˜16.8% O₂), and finally steamed at950° F. for 4 hours. Bottom bed catalyst alphas (hexane crackingactivity) are typically ˜95.

TABLE 2 Bottom Bed Catalyst Formulations 2nd Pt, Metal, 2nd Metal, Pt/M,Catalyst wt % (M) wt % Metal Precursor Molar Ratio X 0.115 None — — — Y0.115 Ag 0.0318 silver nitrate 2/1 Z 0.115 Ag 0.0636 silver nitrate 1/1AA 0.115 Ag 0.1272 silver nitrate 1/2 AB 0.115 Ag 0.1908 silver nitrate1/3 AC 0.115 Ag 0.2543 silver nitrate 1/4 AD 0.100 Ag 0.1106 silvernitrate 1/2 AE 0.100 Ag 0.1659 silver nitrate 1/3 AF 0.100 Ag 0.2212silver nitrate 1/4 AG* 0.100 Ag 0.1659 silver nitrate 1/3 AH 0.075 Ag0.1244 silver nitrate 1/3 AI 0.115 Au 0.1161 Gold chloride 1/1 AJ 0.115Cu 0.0187 copper nitrate 2/1 hemipentahydrate AK 0.115 Cu 0.0375 coppernitrate 1/1 hemipentahydrate AL 0.115 Cu 0.7490 copper nitrate 1/2hemipentahydrate AM 0.115 Cu 0.1124 copper nitrate 1/3 hemipentahydrateAN 0.115 In 0.0677 Indium (III) Nitrate 1/1 hydrate AO 0.115 Pb 0.1221Lead nitrate 1/1 AP 0.115 Rh 0.0607 Rhodium Chloride 1/1 AQ 0.115 Ru0.0596 Ru Chloride 1/1 AR 0.115 Sn 0.0350 tin (II) chloride 2/1 AS 0.115Sn 0.0700 tin (II) chloride 1/1 AT 0.115 Sn 0.1400 tin (II) chloride 1/2AU 0.115 Zn 0.0386 Zinc nitrate 1/1 hexahydrate

Initial Dual-Bed Screening was then conducted, as described below.

Catalysts were tested in a fixed-bed micro unit. A total of 2 grams ofcatalyst (prepared as described above, “as-is”) was loaded intoreactors: 0.5 grams of top bed and 1.5 grams of bottom bed. The reactorpressure was 225 psig (1551 kPa) and the H₂:HC ratio was 1:1. The totalfeed flow rate, expressed as grams feed per gram catalyst per hour(WHSV) was 12 hr−1. The activity of the catalysts was determined as afunction of reactor temperature (660 to 730° F. or 349 to 388° C.). Thetemperature range allowed comparison across a range of ethylbenzeneconversions, from about 55 to 85 wt %. The feed to the reactor containedmostly C8 aromatic hydrocarbons with approximately 16% ethylbenzene. Adetailed analysis of the feed is shown in Table 3. The catalysts werereduced in hydrogen for 1 hour at 400° C. prior to the introduction offeed. No sulfiding was performed. Product analysis occurred usingon-line GC-FID with a 60-M DB-WAX column Saturates in the product (wt %)was used to indicate relative ring saturation activity of the differentcatalyst systems.

The catalysts with 1:1 molar ratio of Pt to the second metal weretested. All metal combinations showed some improvement over Pt only.Indium and tin showed the most reduction in saturates, followed by Cu,Ag, Rh, and Ru. Results for the various two-metal systems studied areshown in FIG. 1.

TABLE 3 Feed Properties Feed Component wt % C5- 0.1 Saturates 0.3Benzene 0.0 Toluene 1.2 Ethylbenzene 16.3 Para Xylene 2.0 Meta Xylene64.7 Ortho Xylene 15.4 Total C9+ 0.0

Secondary Dual-Bed Testing was then performed as described below.

A second series of fixed-bed micro unit testing was performed on selectcatalyst systems including Pt—Sn, Pt—Ag, and Pt—Cu catalysts madeaccording to the present invention. A second GC column (DB-1) was addedto the GC configuration to allow more detailed analysis, includedquantification of the ethylene saturation activity through theethane/ethylene ratio, and yield calculations. A total of 2 grams of thespecified catalyst (“as-is”) was loaded into reactors. The bimetallicsused 0.5 grams of top bed and 1.5 grams of bottom bed. The reactorpressure was 225 psig (1551 kPa) and the H₂:HC ratio was 1:1. The totalfeed flow rate, expressed as grams feed per gram catalyst per hour(WHSV) was 12 hr⁻¹ for all catalysts. The activity of the catalysts wasdetermined as a function of reactor temperature. The feed to the reactorcontained mostly C8 aromatic hydrocarbons with approximately 16%ethylbenzene. A detailed analysis of the feed is shown in Table 4. Thecatalysts were not sulfided. All catalysts tested were de-edged for 24hours at 430° C., H₂:HC ratio of 0.9:1, and pressure 185 psig (1275 kPa)before moving to the temperature scan from 640/650° F. to 745/755° F.Products were analyzed using an on-line GC-FID with a 60-M DB-WAX columnand a DB-1 column (each of the designated columns, DB-WAX and DB-1, perse known in the art and commercially available). Typically the desiredlevel of ethylbenzene conversion (“EBC”) is >70%.

Comparison of certain two-metal systems of the present invention having1:1 Pt:M catalysts for M=Ag, Cu, and Sn are shown in FIGS. 2-4;specifically with respect to ring loss versus ethylbenzene conversion(FIG. 2), ring loss versus temperature (FIG. 3) and molar C2 ratio(ethane/ethylene ratio; FIG. 4).

All the catalysts had low ring loss (low aromatics saturation) and highC2/C2=molar ratio (high ethylene saturation).

TABLE 4 Feed Properties Benzene 0 Toluene 1.2 Saturates 0.14 Ethylbenzene 16.3 Ortho Xylene 15.3 Meta Xylene 65.1 Para Xylene 1.9 TotalC9+ 0.01

Catalyst Preparation EXAMPLE 1 Top-Bed Catalyst Preparation: 0.1 wt % Ptand 1:0 Pt/Ag Molar Ratio

The catalyst used for top-bed catalyst preparation was H-ZSM-5 with 35%silica binder in a 1/16″ (approximately 0.16 cm) cylindrical extrudateform. The H-ZSM-5 was selectivated three times with silicone beforemetal impregnation. An impregnation solution made with 1.41 g oftetraamine platinum nitrate solution (an aqueous solution with 3.55 wt %Pt) and 16.7 g of de-ionized (DI) water was slowly added to 50 g of thecatalyst. The mixture was tumbled thoroughly until completely loose. Themixture was air-dried first at ambient conditions then at 250° F. forfour hours. The mixture was calcined at 660° F. (352° C.) for 3 hourswith a mixture of 40% air and 60% nitrogen at a flow rate of 5vol/vol/min.

EXAMPLE 2 Top-Bed Catalyst Preparation: 0.1 wt % Pt and 1:1 Pt/Ag MolarRatio

The same procedure described in Example 1 was followed except that theimpregnation solution was made with the composition below:

-   -   1.41 g of tetraamine platinum nitrate solution (an aqueous        solution with 3.55 wt % Pt)    -   0.0446 g of chemical grade silver nitrate, solid    -   16.7 g of DI water

EXAMPLE 3 Top-Bed Catalyst Preparation: 0.1 wt % Pt and 1:2 Pt/Ag MolarRatio

The same procedure described in Example 1 was followed except that thecatalyst used was 100 g and the impregnation solution was made with thecomposition below:

-   -   2.83 g of tetraamine platinum nitrate solution (an aqueous        solution with 3.55 wt % Pt)    -   0.175 g of chemical grade silver nitrate, solid    -   26.2 g of DI water

EXAMPLE 4 Top-Bed Catalyst Preparation: 0.1 wt % Pt and 1:3 Pt/Ag MolarRatio

The same procedure described in Example 3 was followed except that theimpregnation solution was made with the composition below:

-   -   2.83 g of tetraamine platinum nitrate solution (an aqueous        solution with 3.55 wt % Pt)    -   0.265 g of chemical grade silver nitrate, solid    -   26.2 g of DI water

EXAMPLE 5 Top-Bed Catalyst Preparation: 0.1 wt % Pt and 1:4 Pt/Ag MolarRatio

The same procedure described in Example 3 was followed except that theimpregnation solution was made with the composition below:

-   -   2.83 g of tetraamine platinum nitrate solution (an aqueous        solution with 3.55 wt % Pt)    -   0.36 g of chemical grade silver nitrate, solid    -   27.5 g of DI water

EXAMPLE 6 Bottom-Bed Catalyst Preparation: 0.115 wt % Pt and 1:0 Pt/AgMolar Ratio

Extrusion: A mixture of 400 g of H-ZSM-5 crystal and 100 g Versal 300was mulled thoroughly in a Muller. A 271 g of DI water was added to themixture while mulling. An impregnation solution made with the followingcomposition was added to the mixture while mulling.

-   -   16.22 g of tetraamine platinum nitrate solution (an aqueous        solution with 3.55 wt % Pt)    -   120 g of DI water

The metal-impregnated mixture was extruded with a 1/16″ cylinder dieplate. The extrudate was dried at 250° F.

Calcination: The extrudate was heated in flowing nitrogen (5vol/vol/min) at 150° F./h to 900° F. (482° C.), hold at 900° F. for 3hr. While at 900° F., the gas mixture was changed to 0.25 vol/vol/minair+4.75 vol/vol/min nitrogen, hold for 30 min; 0.50 vol/vol/minair+4.50 vol/vol/min nitrogen, hold for 30 min; 1.0 vol/vol/min air+4.0vol/vol/min nitrogen, hold for 30 min; 2.0 vol/vol/min air+3.0vol/vol/min nitrogen, hold for 30 min. The temperature was increased at150° F./h (83.3° C./h) to 1000° F. (538° C.). Once stabilized 1000° F.,the gas mixture was changed to 4 vol/vol/min air+1 vol/vol/min nitrogenand hold for 6 hours. Cool down to ambient conditions and discharge.

Steaming:

The calcined catalyst was heated in flowing nitrogen (5 vol/vol/min) at150° F./h to 900° F., hold at 900° F. for 30 min. Switch to steam over a30 min period. Increase temperature at 150° F./h to 950° F. (510° C.),and then hold for 4 hours in 100% steam. Cool down in air and discharge.

EXAMPLE 7 Bottom-Bed Catalyst Preparation: 0.115 wt % Pt and 1:1 Pt/AgMolar Ratio

The three-step procedure described in Example 6 was followed except thatthe impregnation solution was made with the composition below:

-   -   16.22 g of tetraamine platinum nitrate solution (an aqueous        solution with 3.55 wt % Pt)    -   0.502 g of chemical grade silver nitrate, solid    -   120 g of DI water

EXAMPLE 8 Bottom-Bed Catalyst Preparation: 0.115 wt % Pt and 1:2 Pt/AgMolar Ratio

The three-step procedure described in Example 6 was followed except thatthe impregnation solution was made with the composition below:

-   -   16.22 g of tetraamine platinum nitrate solution (an aqueous        solution with 3.55 wt % Pt)    -   1.004 g of chemical grade silver nitrate, solid    -   120 g of DI water

EXAMPLE 9 Bottom-Bed Catalyst Preparation: 0.1 wt % Pt and 1:3 Pt/AgMolar Ratio

The three-step procedure described in Example 6 was followed except thatthe impregnation solution was made with the composition below:

-   -   14.10 g of tetraamine platinum nitrate solution (an aqueous        solution with 3.55 wt % Pt)    -   1.310 g of chemical grade silver nitrate, solid    -   120 g of DI water

EXAMPLE 10 Bottom-Bed Catalyst Preparation: 0.1 wt % Pt and 1:4 Pt/AgMolar Ratio

The three-step procedure described in Example 6 was followed except thatthe impregnation solution was made with the composition below:

-   -   14.10 g of tetraamine platinum nitrate solution (an aqueous        solution with 3.55 wt % Pt)    -   1.746 g of chemical grade silver nitrate, solid    -   120 g of DI water

Catalyst evaluation in fixed-bed micro units is described below.

EXAMPLE 11 Evaluation of Pt/ZSM-5 Catalysts With 1:0 Pt/Ag

A fixed bed reactor with ⅜″ external diameter was used for theevaluation. The reactor was equipped with a ⅛″ diameter thermal well tomonitor reactor temperature at the center of the catalyst bed. Thecatalysts in the shape of cylindrical 1/16″ extrudate were loaded to thereactor based on the catalyst information provided in the table below.

Pt/Ag Catalyst Pt, wt % (mole ratio) Cat Wt Top Bed, Example 1 0.100 1:00.5 g Bottom Bed, Example 6 0.115 1:0 1.5 g

The reactor pressure was set at 225 psig with a steady flow of H₂ at 92cc/min. The reactor temperature was increased at 0.833° C./min to 200°C., and held at 200° C. for 16 hours. The temperature was furtherincrease at 0.833° C./min to 380° C., and held at 380° C. for 3 hours.The feed was introduced at 27.6 cc/hr (12 WHSV). This feed rate wasmaintained through the entire run. The feed composition is show in thetable below. The feed density was 0.87 g/cc.

Feed Component Weight, % Toluene 0.59 Ethylbenzene 14.75 O-Xylene 18.24M-Xylene 62.72 P-Xylene 2.51 Propylbenzene 0.01 Isopropylbenzene 0.041-Methy-3-ethylbenzene 0.01 1-Methy-4-ethylbenzene 0.011,4-Diethylbenzene 0.01 Other C10 Aromatics 0.01 C11 Aromatics 1.13Total 100.00

The reactor pressure was then decreased to 185 psig, the H₂ flow wasreduced to 82 cc/min, and reactor temperature was increase at 0.833°C./min to 430° C., and held at 430° C. for 24 hours.

The reactor pressure was increased to 225 psig, the H₂ flow wasincreased to 92 cc/min, and reactor temperature was reduced to 340° C.,and held for 12 hours at 340° C. for data collection by online GCanalysis. The reactor temperature was further increased at 0.833° C./minto 355, 370, 385, and 400° C. consecutively and held for 12 hours ateach temperature setting for data collection by online GC analysis. Theresults are compared with the rest of the catalysts in the DiscussionsSection.

EXAMPLE 12 Evaluation Pt/ZSM-5 Catalysts With 1:1 Pt/Ag

The same procedure described in Example 1 was followed to evaluate thesecond set of catalysts described in the table below. The results arecompared with the rest of the catalysts in the Discussions section.

Pt/Ag Catalyst Pt, wt % (mole ratio) Cat Wt Top Bed, Example 2 0.100 1:10.5 g Bottom Bed, Example 7 0.115 1:1 1.5 g

EXAMPLE 13 Evaluation of Pt/ZSM-5 Catalysts With 1:2 Pt/Ag

The same procedure described in Example 1 was followed to evaluate thethird set of catalysts described in the following table. The results arecompared with the rest of the catalysts in the Discussions section.

Pt/Ag Catalyst Pt, wt % (mole ratio) Cat Wt Top Bed, Example 3 0.100 1:20.5 g Bottom Bed, Example 8 0.115 1:2 1.5 g

EXAMPLE 14 Evaluation of Pt/ZSM-5 Catalysts With 1:3 Pt/Ag

The same procedure described in Example 1 was followed to evaluate thefourth set of catalysts described in the following table. The resultsare compared with the rest of the catalysts in the Discussions section.

Pt/Ag Catalyst Pt, wt % (mole ratio) Cat Wt Top Bed, Example 4 0.100 1:30.5 g Bottom Bed, Example 9 0.100 1:3 1.5 g

EXAMPLE 15 Evaluation of Pt/ZSM-5 Catalysts With 1:4 Pt/Ag

The same procedure described in Example 1 was followed to evaluate thefifth set of catalysts described in the following table. The results arecompared with the rest of the catalysts in the Discussions section.

Pt/Ag Catalyst Pt, wt % (mole ratio) Cat Wt Top Bed, Example 5 0.100 1:40.5 g Bottom Bed, Example 10 0.100 1:4 1.5 g

Table 5 shows the micro unit results at EB conversion around 75%. Whencompared with the Ag-free catalyst, all Ag-containing catalysts hadreduced production of C⁵⁻, C₆ ⁻ non-aromatics, and C₉ ⁺ aromatics aswell as lower xylene loss and ring loss. All Ag-containing catalystsalso had higher benzene selectivity from EB.

TABLE 5 Comparison of Catalyst Performance Pt loading, wt % Top bed/btmbed 0.100/0.115 0.100/0.115 0.100/0.115 0.100/0.100 0.100/0.100 Pt/AgMolar ratio (abbreviation) 1:0 (0x Ag) 1:1 (1x Ag) 1:2 (2x Ag) 1:3 (3xAg) 1:4 (4x Ag) Temperature, ° C. 386 384 384 397 396 Temperature, ° F.726 724 724 746 746 Pressure, psig 233 233 234 225 218 Feed flow rate,WHSV 12 12 12 12 12 H₂/HC Molar ratio 1 1 1 1 1 Time on stream, hr 70.574.5 70.5 82.5 82.5 EB Conversion, % 74.1 77.6 75.5 78.9 73.8 ProductDistribution, wt % (feed) H₂ 0.00 −0.39 −0.25 −0.23 −0.22 −0.20 C₅ ⁻(light hydrocarbons) 0.00 4.37 3.50 3.27 3.36 3.08 C₆ ⁺ Non-aromatics0.00 1.11 0.42 0.41 0.32 0.31 C₉ ⁺ Aromatics 1.19 1.59 0.88 0.92 1.021.08 Benzene 0.00 6.45 7.90 7.45 8.08 7.47 Toluene 0.59 2.57 2.22 2.102.45 2.28 p-Xylene 2.51 19.22 19.30 19.40 19.70 19.67 o-Xylene 18.2418.20 18.51 18.58 18.69 18.72 m-Xylene 62.72 43.05 44.20 44.47 43.4943.71 EB 14.75 3.83 3.31 3.62 3.10 3.87 Total 100.0 100.0 100.0 100.0100.0 100.0 p-Xylene + m-Xylene 65.23 62.27 63.50 63.87 63.19 63.39Total xylenes 83.49 80.47 82.02 82.45 81.89 82.11 C₂/C₂ ⁼, Molar ratio2510 2746 2592 2666 1242 Xylene loss ¹, wt % 3.59 1.74 1.22 1.89 1.62Ring loss ², mole % 2.41 0.40 0.35 0.11 0.09 PXAE ³ 101.5 99.8 99.7102.5 102.0 Benzene Sel from EB ⁴, % 79.9 93.5 90.7 93.9 93.0 ¹ Xyleneloss = 100 × (feed xylene − product xylene)/feed xylene. ² Ring loss =100 × (total aromatic carbon in feed − total aromatic carbon inproducts)/total aromatic carbon in feed. ³ p-Xylene approachingequilibrium. ⁴ Benzene Selectivity from EB = 100 × (Product benzene −Feed benzene)/(Feed EB − Product EB).

While the illustrative embodiments of the invention have been describedwith particularity, it will be understood that various othermodifications will be apparent to and can be readily made by thoseskilled in the art without departing from the spirit and scope of theinvention.

Trade names used herein are indicated by a ™ symbol or ® symbol,indicating that the names may be protected by certain trademark rights,e.g., they may be registered trademarks in various jurisdictions. Allpatents and patent applications, test procedures (such as ASTM methods,UL methods, and the like), and other documents cited herein are fullyincorporated by reference to the extent such disclosure is notinconsistent with this invention and for all jurisdictions in which suchincorporation is permitted. When numerical lower limits and numericalupper limits are listed herein, ranges from any lower limit to any upperlimit are contemplated.

What is claimed is:
 1. A catalyst system comprising a first bedincluding at least one first metal selected from Groups 7-10, and atleast one second metal selected from silver, copper, ruthenium, indiumand tin, dispersed on ZSM-5, and a second bed including at least onefirst metal selected from Groups 7-10, and at least one second metalselected from silver, copper, ruthenium, indium and tin, dispersed onZSM-5, further characterized in that ZSM-5 in the first bed issilicon-selectivated and ZSM-5 in the second bed is notsilicon-selectivated.
 2. The catalyst system according to claim 1,wherein at least one of said first bed and said second bed furthercomprises a support selected independently from at least one of alumina,silica, aluminosilicates, clay, and combinations thereof
 3. The catalystsystem according to claim 1, further characterized as including a firstbed comprising a catalyst made by adding at least one first metalselected from Groups 7-10 and at least one second metal selected fromsilver, copper, ruthenium, indium and tin by the incipient wetnesstechnique to a silicone-selectivated ZSM-5, followed by drying andcalcination, and a second bed comprising a catalyst made by combining ametal solution comprising at least one first metal selected from Groups7-10 and at least one second metal selected from silver, copper,ruthenium, indium and tin, and water and a non-selectivated ZSM-5molecular sieve and mulling with a support, followed by extrusion,drying, calcination, and steaming.
 4. The catalyst system according toclaim 1, wherein said first bed comprises at least one first metalselected from platinum, rhenium, and mixtures thereof, and at least onesecond metal selected from silver, copper, tin, and mixtures thereof,and said second bed comprises at least one first metal selected fromplatinum, rhenium, and mixtures thereof, and at least one second metalselected from silver, copper, tin, and mixtures thereof
 5. The catalystsystem according to claim 1, wherein said first bed comprises at leastone first metal selected from platinum, rhenium, and mixtures thereof,and at least one second metal selected from silver, tin, and mixturesthereof, and said second bed comprises at least one first metal selectedfrom platinum, rhenium, and mixtures thereof, and at least one secondmetal selected from silver, tin, and mixtures thereof
 6. The catalystsystem according to claim 1, wherein said first metal in said first bedis rhenium and said first metal in said second bed is rhenium.
 7. Thecatalyst system according to claim 1, wherein said second metal in saidfirst bed is silver and said second metal in said second bed is silver.8. The catalyst system according to claim 1, wherein said second metalin said first bed is tin and said second metal in said second bed istin.
 9. A process comprising contacting a paraxylene-depleted C8aromatic hydrocarbon feed including ethylbenzene and xylene isomers witha catalyst system according to any one of the preceding claims, toprovide a final product comprising an increased amount of paraxylene anddecreased amount of ethylbenzene when compared with said feed.